Abstract

We consider a composite system consisting of two identical quantum systems. Initially the two subsystems are separated so that the initial state of the system is a product state, and we assume that both factors are canonical equilibrium states, but at different temperatures. Then the two systems are thermally coupled which we describe by a certain class of interactions. We show ‐ at least numerically ‐ the following version of the second law: heat flows always from the hotter to the colder system.